It is known that uninterruptible power supplies or UPSs are apparatuses capable of supplying electric power to connected user devices (output loads) when the main power supply line fails.
These apparatuses in practice are connected electrically to two separate and distinct power sources: on one side, the ordinary electrical mains and, on the other side, one or more batteries arranged in a series or parallel configuration.
However, known uninterruptible power supplies draw the power required by user devices from only one of these power sources and in particular from the electrical mains in normal operating conditions and from the batteries in emergency conditions.
The occurrence of these emergency conditions is often due to the fact that the ordinary electrical mains has supply limitations dictated by the user contracts, by the sizing of the supply facilities, and by the maximum power that can be delivered to a geographical area by the provider (ENEL).
These limitations, for example, prescribe that the absorbed power for home users is lower than 3.2 kW; exceeding this level entails the payment of higher rates by the user and in some cases entails disconnection due to overload of the mains supply and the beginning of the supply of power on the part of the batteries.
These traditional types of apparatus are not free from drawbacks, including the fact that in emergency conditions they are unable to supply a particularly large amount of energy for a very long and continuous period of time. In such circumstances they must in fact provide on their own the total energy requirements of the user devices and therefore they tend to be drained rapidly; the higher the electric power to be delivered, the faster they are drained.
Further, in order to be able to provide better power and duration performance, these batteries must be sized appropriately, installing a larger and bulkier group, consequently entailing considerably higher installation and maintenance costs.
Another disadvantage of conventional apparatuses is that if the contracted supply limit is exceeded without the electrical mains failing, the user is not aware of the considerable energy consumption and of the particularly high costs linked to this consumption; therefore, he is unlikely to reduce his electric power demand.